1. Field of the Invention
The present invention relates to a negative feed-back amplifier and more particularly to a negative feed-back amplifier that can be suitably used in a video IF (Intermediate Frequency) amplifier installed on a video signal circuit of a television and to a method for negative feed-back.
The present application claims priority of Japanese Patent Application No. 2000-166868 filed on Jun. 2, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
As shown in FIG. 8, a picture signal related circuit of a television is provided with a tuner 1, a band-pass filter (hereinafter referred to as a BPF 2), a local oscillator 3, a mixer 4, an immediate frequency preamplifier (hereinafter referred to as an IF frequency preamplifier 5), a condenser 6, and an intermediate frequency main amplifier (hereinafter referred simply to as an IF main amplifier 7). The IF preamplifier 5, the condenser 6, and a negative feed-back amplifier (the IF main amplifier 7) make up a video IF amplifier.
FIG. 9 is a circuit diagram showing electrical configurations of the IF main amplifier 7 shown in FIG. 8. The IF main amplifier 7 is provided with a bipolar transistor (hereinafter referred to a transistor 7a), a resistor 7b, a resistor 7c, a transistor 7d, a transistor 7e, a resistor 7f, a resistor 7g, and a resistor 7h. 
In the picture signal related circuit, a receiving signal received by an antenna (not shown) is input to the tuner 1 and a video intermediate frequency signal V1 is output from the tuner 1. The video intermediate frequency signal V1 is then input to the BPF 2 and an output signal V2 in a predetermined frequency band is output. The output signal V2 is mixed with an output signal V3 fed from the local oscillator 3 by the mixer 4 and an output signal V4 is output from the mixer 4. The output signal V4 is amplified by the IF preamplifier 5 and an output signal V5 is output from the IF preamplifier 5. The output signal V5 is input to the IF main amplifier 7 through the condenser 6 and amplified and then a video detecting signal xe2x80x9cOUTxe2x80x9d is output from the IF main amplifier 7. The video detecting signal xe2x80x9cOUTxe2x80x9d is transmitted to a video detecting amplifier (not shown). In this case, as shown in FIG. 9, an output impedance of the IF main amplifier 7 is determined by the resistor 7f and a resistance of the resistor 7f is set to a value that does not cause the video detecting signal xe2x80x9cOUTxe2x80x9d to occur, with considerations given to a state of matching with a characteristic impedance of a transmission line connected to an output of the IF main amplifier 7 and with an input impedance of the video detecting amplifier (not shown). Moreover, the video detecting signal xe2x80x9cOUTxe2x80x9d is fed back to a base of the transistor 7a through the resistor 7h. 
However, in the conventional IF main amplifier 7, since, depending on set values of the resistors 7b, 7c, 7f, and 7g, a base potential of the transistor 7a becomes too high and a dynamic range in a first stage becomes small; if an input signal xe2x80x9cINxe2x80x9d is large, in some cases, a distortion of the video detecting signal xe2x80x9cOUTxe2x80x9d increases. This presents a problem in that an image quality of the television deteriorates.
In view of the above, it is an object of the present invention to provide a negative feed-back amplifier and a method for negative feed-back which are capable of preventing a distortion of a signal even if an input signal is large.
According to a first aspect of the present invention, there is provided a negative feed-back amplifier including;
a push-pull amplifier made up of bipolar transistors and used to amplify an input signal and to output an output signal; and
a negative feed-back circuit used to bias a base of a bipolar transistor at an initial stage of the push-pull amplifier by using a negative feed-back signal produced by attenuation of the output signal at a predetermined feedback ratio.
According to a second aspect of the present invention, there is provided a negative feed-back amplifier including:
a push-pull amplifier made up of bipolar transistors and used to amplify an input signal and to output an output signal; and
a negative feed-back circuit used to bias a base of a bipolar transistor at an initial stage of the push-pull amplifier by using a negative feed-back signal produced by attenuation of a signal corresponding to the output signal at a predetermined feedback ratio.
According to a third aspect of the present invention, there is provided a negative feed-back amplifier including:
a phase inverter amplifier used to amplify an input signal and to produce a reversed signal of a phase opposite to the input signal and to produce a non-reversed signal being in phase with the input signal;
a push-pull output circuit used to produce an output signal of a phase opposite to the input signal by inputting the reversed signal at high input impedance and outputting the reversed signal at low output impedance and outputting the non-reversed signal in a reversed state;
a negative feed-back circuit used to produce a negative feed-back signal by inputting the output signal and to add the negative feed-back signal to the input signal; and
wherein the negative feed-back circuit is provided with a feedback ratio setting circuit used to attenuate the output signal at a predetermined feedback ratio and to produce the negative feed-back signal.
According to a fourth aspect of the present invention, there is provided a negative feed-back amplifier including:
a phase inverter amplifier used to amplify an input signal and to produce a reversed signal of a phase opposite to the input signal and to produce a non-reversed signal being in phase with the input signal;
a push-pull output circuit used to produce an output signal of a phase opposite to the input signal by inputting the reversed signal at high input impedance and outputting the reversed signal at low output impedance and outputting the non-reversed signal in a reversed state;
a negative feed-back circuit used to produce a negative feed-back signal by inputting the output signal and to add the negative feed-back signal to the input signal; and
wherein the negative feed-back circuit has an impedance converter used to produce a feed-back signal by inputting the output signal at high input impedance and outputting the output signal at low output impedance and a feed-back ratio setting circuit used to produce the negative feed-back signal by attenuating the feed-back signal at a predetermined feed-back ratio.
According to a fifth aspect of the present invention, there is provided a negative feed-back amplifier including:
a phase inverter amplifier used to amplify an input signal and to produce a reversed signal of a phase opposite to the input signal;
a push-pull output circuit used to produce an output signal of a phase opposite to the input signal by inputting the reversed signal at high input impedance and outputting the reversed signal at low output impedance;
a negative feed-back circuit used to produce a negative feed-back signal by inputting the output signal and to add the negative feed-back signal to the input signal; and
wherein the negative feed-back circuit is provided with a feedback ratio setting circuit used to attenuate the output signal at a predetermined feedback ratio and to produce the negative feed-back signal.
According to a sixth aspect of the present invention, there is provided a negative feed-back amplifier including:
a phase inverter amplifier used to amplify an input signal and to produce a reversed signal of a phase opposite to the input signal;
a push-pull output circuit used to produce an output signal of a phase opposite to the input signal by inputting the reversed signal at high input impedance and outputting the reversed signal at low output impedance;
a negative feed-back circuit used to produce a negative feed-back signal by inputting the output signal and to add the negative feed-back signal to the input signal; and
wherein the negative feed-back circuit has an impedance converter used to produce a feed-back signal by inputting the output signal at high input impedance and outputting the output signal at low output impedance and a feed-back ratio setting circuit used to produce the negative feed-back signal by attenuating the feed-back signal at a predetermined feed-back ratio.
In the foregoing, a preferable mode is one wherein the impedance converter is made up of an emitter follower of a bipolar transistor or a source follower of an FET (Field Effect Transistor).
According to a seventh aspect of the present invention, there is provided a negative feed-back method including:
a step of biasing a base of a bipolar transistor at an initial stage of a push-pull amplifier by using a negative feed-back signal which is produced by attenuation of an output signal at a predetermined feedback ratio in the push-pull amplifier made up of bipolar transistors and used to amplify an input signal and to output the output signal.
According to an eighth aspect of the present invention, there is provided a negative feed-back method including:
a step of biasing a base of a bipolar transistor at an initial stage of a push-pull amplifier by using a negative feed-back signal which is produced by attenuation of a signal corresponding to an output signal at a predetermined feedback ratio in the push-pull amplifier made up of bipolar transistors and used to amplify an input signal and to output the output signal.
With the above configuration, the output signal from the push-pull amplifier is attenuated by a feedback ratio setting circuit and is fed back to a base of a phase inverter amplifier, thus allowing proper setting of a base bias voltage of the bipolar transistor. This enables a dynamic range of the phase inverter amplifier to be made larger and distortion of the output signal to be reduced and, therefore, when this negative feed-back amplifier is used as a video IF amplifier in televisions, it serves to improve an image quality of the televisions.
With another configuration, the output signal is input to an impedance converter and, as a result, a load is taken off an output circuit and distortion of output signals can be reduced.
With still another configuration, the negative feed-back circuit is mounted independently of the output circuit and, therefore, even if feedback ratio is increased, the bipolar transistor of the output circuit is not easily saturated, thus serving to increase load driving capability and to make calibration of output impedance easier.
With still another configuration, the output circuit is made up of a push-pull type circuit in which a reversed signal is input at high input impedance and output at low output impedance, thus making it possible to make up a comparatively simpler negative feed-back amplifier.